<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        "use strict";
        // 先定义 let username，后使用 赋值
        // Uncaught ReferenceError: username is not defined
        
        // 任何模式下 都应该开启严格模式
        let username;
        username = "lucy";
        console.log(username);
    </script>
</body>
</html>